(1) Field of the Invention
The present invention relates to a method and apparatus for carrying out a loopback test in a data communication system and, more particularly, to a method for carrying out a loopback test in a data communication system which is comprised of many data communication stations connected in a row by transmission lines, that is the so-called multilevel data communication system.
The data communication system is basically comprised of a first data communication station, a second data communication station and a transmission line connecting the two. The data communication system cannot carry out normal operation when trouble occurs anywhere in the system. Generally, a loopback test is carried out in the system to find the location where the trouble has occurred. Also, the loopback test is carried out in the system to check the area of distribution of noise. The loopback test is generally carried out by comparing two signals. The first signal is a test signal transmitted from a first data communication station and second signal is a test signal which has been transmitted from the first data communication station to a second data communication station and has been returned therefrom to the first data communication station. If the two signals coincide with each other, no trouble exists between the first and second data communication stations. In contrast, if the two signals do not coincide with each other, some trouble exists therebetween.
(2) Description of the Prior Art
In the prior art, two methods for carrying out the loopback test are known. In a first known method, a test channel is employed in the system. This test channel exists independently of the usual information data channel. Control data for carrying out the loopback test is transmitted on the newly employed test channel. In a second known method a control signal for carrying out the loopback test is composed of both a tone signal and a control data signal. The tone signal is, for example a sine wave signal having a fixed frequency of less than 300 Hz or greater than 3,400 Hz in the case where the transmission line is formed by an ordinary telephone line. The tone signal is useful for indicating that the forthcoming signal is not the usual information data, but the control data signal for carrying out the loopback test.
However, each of the above described two known methods have disadvantages. The first method has the following disadvantages: a first disadvantage is that the new control circuit for carrying out the loopback test has to be connected in parallel to each data communication station; a second disadvantage is that it is hard to allot a channel for the test in the usual information data channels, especially when there is no extra channel among the information data channels; third disadvantage is that the first method can not be applied to the multilevel data communication system unless a modem for the test channel is mounted in each data communication station. On the other hand, the second method has the following disadvantages: a first disadvantage is that the tone signal can be transferred only to two adjacent data communication stations connected by a transmission line, for example a telephone line; if adjacent data communication stations are connected by cables, and the two stations are modems, or a modem and a time division multiplexer, the tone signal can not be transferred on the cables. This is because the tone signal is not a digital interface signal but an analog signal; a second disadvantage is that this second method can not be applied to a multilevel data communication system unless an appropriate interface means for the tone signal is mounted in each data communication station.